1. Field of the Invention
The present invention relates to a particulate matter concentration measuring apparatus configured to measure the concentration of particulate matter (PM) included in exhaust gas.
2. Discussion of the Background
FIG. 1 shows a known particulate matter concentration measuring apparatus 20PM (PM sensor), as described in EP1916394A1, capable of detecting the concentration of particulate matter (PM) including carbon (C) as a main component in exhaust gas of a diesel engine. As shown in FIG. 1, the particulate matter concentration measuring apparatus 20PM includes an exhaust line 21, a secondary exhaust line 21A branching from the exhaust line 21, a particulate matter detection filter 22A provided in the secondary exhaust line 21A, and a differential pressure sensor 22B measuring differential pressure between inlet and outlet of the particulate matter detection filter 22A. Further, in the secondary exhaust line 21A, a flow rate meter 24 and a temperature sensor T1 are provided. Further, in the particulate matter detection filter 22A, a heater 22H is provided.
According to Patent EP1916394A1, in the particulate matter concentration measuring apparatus 20PM, a differential pressure ΔP across the particulate matter detection filter 22A, a temperature T of exhaust gas in the secondary exhaust line 21A, and a flow amount Q2 of exhaust gas in the secondary exhaust line 21A are measured. Then, based on the measured values of the differential pressure ΔP, the temperature T of the exhaust gas, and the flow amount Q2 of the exhaust gas, a value of PM [g/h] which is the mass of particulate matter (PM) collected on the particulate matter detection filter 22A per unit time is calculated. Based on the PM [g/h] which is the mass of particulate matter (PM), a value of PMconc [g/cm3] which is the concentration of particulate matter (PM) in the exhaust gas is calculated. In this case, if a large amount of particulate matter (PM) is accumulated on the particulate matter detection filter 22A, detection accuracy of the differential pressure ΔP may be more likely to be degraded. To overcome the problem, in the particulate matter concentration measurement according to EP1916394A1, a heater 22H is provided so as to burn and remove particulate matter (PM) when the particulate matter (PM) is accumulated on the particulate matter detection filter 22A to some extent.
Further, EP1916394A1 discloses a Diesel Particulate Filter (DPF) 22 made of porous ceramic material and provided in the exhaust line 21 of an exhaust gas purification apparatus 20. Further, the secondary exhaust line 21A is connected to the exhaust line 21 on the upstream side of the Diesel Particulate Filter (DPF) 22, and based on a value of PMconc [g/m3] which is concentration of particulate matter (PM) in exhaust gas and an engine operating status or a flow amount Q1 of the exhaust gas introduced into the Diesel Particulate Filter (DPF) 22, a value of PMenter-full-filter [g/h] which is the mass of particulate matter (PM) introduced into the Diesel Particulate Filter (DPF) 22 is calculated.
Similar to the particulate matter detection filter 22A, in the Diesel Particulate Filter (DPF) 22, the collected particulate matter (PM) is likely to be gradually accumulated with continuous use of the Diesel Particulate Filter (DPF) 22. If the accumulation of the particulate matter (PM) in the Diesel Particulate Filter (DPF) 22 is ignored, a pressure caused by exhaust gas may become too high, which may lower fuel economy and/or cause engine damage.
To avoid the problems, in the exhaust gas purification apparatus 20 using the Diesel Particulate Filter (DPF) 22, the accumulated particulate matter (PM) is burned and removed at regular intervals in the Diesel Particulate Filter (DPF) 22 so as to regenerate the Diesel Particulate Filter (DPF) 22.
To regenerate the Diesel Particulate Filter (DPF) 22, high-temperature exhaust gas is introduced into the Diesel Particulate Filter (DPF) 22 so that the accumulated particulate matter (PM) can be burned and removed.
EP1916394A1 describes that, by obtaining the value of PMenter-full-filter [g/h] which is the mass of particulate matter (PM) collected on the Diesel Particulate Filter (DPF) 22, it becomes possible to accurately determine whether an amount of particulate matter (PM) having been actually collected on the Diesel Particulate Filter (DPF) 22 exceeds a predetermined threshold value which necessitates the regeneration of the Diesel Particulate Filter (DPF) 22.